Color television picture tube of the single electron gun type

ABSTRACT

A color television picture tube comprises a single electron gun including a plurality of cathodes arranged in a parallel relationship. A control grid is disposed near the cathodes and formed with a plurality of vertically oriented elongated slits. Each slit is disposed in a position corresponding to the positions of said cathodes. The electron beams, emitted from the electron gun, cross over and focus on a shadow mask. A postdeflection focusing of the electron beams forms an image on a phosphorescent screen. A plurality of phosphor dots on the television tube are arranged on positions in which the images are formed.

United States Patent [72] Inventor Keuiro Takayanagi Kamamira, Japan [21 App]. No. 829,260

[22] Filed June 2,1969

[45] Patented Sept. 7,1971

Victor Company of Japan, Limited Kanagawa-ku, Yokohama, Japan [73] Assignee [32] Priority June 4, 1968 1 J p 43/138,152

[54] COLOR TELEVISION PICTURE TUBE OF THE Assistant Examiner--Joesph G. Baxter Att0rney-Louis Bernat ABSTRACT: A color television picture tube comprises a single electron gun including a plurality of cathodes arranged in a parallel relationship. A control grid is disposed near the cathodes and formed with a plurality of vertically oriented elongated slits. Each slit is disposed in a position corresponding to the positions of said cathodes. The electron beams, emitted from the electron gun, cross over and focus on a shadow mask. A postdeflection focusing of the electron beams forms an image on a phosphorescent screen. A plurality of phosphor dots on the television tube are arranged on positions in which the images are formed.

PATENTEU SEP 7 l9?! SHEET 1 OF 2 INVENTOR KENJIRO TAKAYANAGI BY (/4 ATTCSRNEY PATENTEU SEP 7 I97! 3.603839 sum 2 [IF 2 INVENTOR KENJIRO TAK AYAN A6 I ATTORNEY COLOR TELEVISION PICTURE TUBE OF THE SINGLE ELECTRON GUN TYPE This invention relates to color television picture tubes, and more particularly to color television tubes of the postdeflection-focusing type having a single electron gun.

The most generally used color television picture tube is the so-called three gun, shadow-mask, color picture tube which was developed by the Radio Corporation of America. This tube includes three electron guns, each disposed on an individually associated corner of an equilateral triangle.

Several problems are encountered in this three-gun type of picture tube.

Since the three electron guns are mounted independently of one another, the spacing between the electron beams emitted from the electron guns is limited by the diameter of electrodes of the three electron guns. It is thus impossible to reduce the spacing below a given level.

This makes it necessary to adjust static and dynamic convergences so that each of the three electron beams may be accurately positioned to strike the phosphor dots, as desired and so that the three beams may simultaneously pass through the same aperture in the shadow-mask. The picture tube must have a complicated construction if such adjustments are to be effected. The method of making these adjustments is also complicated and requires a considerable skill. Moreover, the performance of the picture tube lacks stability.

The shadow-mask used with this type of picture tube has apertures of a relatively small diameter. The ratio of the electron beams passing through the apertures to those that fail to pass therethrough has a low value of about 15 percent. As a result, the brightness of the color picture is lower than the brightness of a comparable black-and-white picture tube. An attempt to increase the brightness of the picture is inevitably accompanied by a marked lowering of resolution, due to blooming, because a large amount of the electron beams are required.

The chromatron tube has an arrangement of electrodes forming a single electron gun of the postdeflection focusing type. The phosphor screens of this type of picture tube are coated with colored phosphors of red, green and blue colors arranged in a linear pattern in the indicated order. A pair of grille electrodes are mounted in a position spaced apart from the phosphor screens toward the electron gun. Each of these electrodes includes a number of thin multifrlament metal wires which are electrically insulated from one another.

An alternating voltage is applied between these grille electrodes so that a single electron beam can scan the phosphors of different colors in chronological sequence.

In this type of picture tube, the alternating voltage must have a high frequency (for example, the subcarrier frequency of a color video signal is 3.58 megahertz) in order to prevent the occurrence of color flicker in the colored image displayed on the screen. This results in an increase of reactive power due to The electrostatic capacity between the grille electrodes. This also makes it necessary to increase the output of a generator of this alternating voltage. Accordingly, this arrangement increases the cost of the circuit.

In addition, this picture tube has a disadvantage in that the voltage of the generator is induced in the other circuits, and that induced voltage, in turn, causes failures.

The present invention overcomes the aforementioned disadvantages of prior art color television picture tubes.

The electron gun used in the present invention comprises a plurality of cathodes arranged in a parallel relationship. A control grid is disposed near the cathodes. This grid has vertically oriented, elongated slits disposed in positions corresponding to the electron-beam-emitting faces of the cathodes. A plurality of electrodes constitutes a plurality of electron lenses for converging the electron beams emitted from the cathodes. The invention uses a shadow-mask electrode as a convergence electrode.

The red, green and blue components of a video signal are applied, for example, to the corresponding cathodes. The electron beams emanating from the cathodes disposed in juxtaposed relation with the control grid pass through respective slits in the control grid. A first election lease causes the images of the electron beams formed on the faces of the slits to be a second electron lens. Then, the electron beam images formed in the second electron lens are formed on the phosphor dot screen by the convergence electrode. Thus, a single electron beam is composed of a plurality of portions, each having an electron density corresponding to that of each primary color signal voltage which strikes a corresponding phosphor dot on the screen. It will be evident that if primary color phosphor dots are arranged on the phosphor dot screen in such a manner that they correspond in position to the respective primary color portions of the electron beam; then it is possible to reproduce a color video signal as a color picture.

The single electron beam emitted from a single-electron gun comprises virtually three electron beams of the three primary colors. With the arrangement of this invention, it is possible to eliminate the adjustments of static and dynamic convergences. The Color picture tube is less complex.

The present invention makes it possible to simplify the construction of the color television picture tube and to readily effect adjustments. The present invention obviates the use of a voltage regulator for maintaining the voltage of the phosphor dot screen constant. The invention also makes it possible to use a deflection device which is as simple in construction, as that used with a black-and-white picture tube.

In a postdeflection focusing system, the electron beams passing through the second electron lens are very small in sectional area and conductive to increased deflection sensitivity of the color television picture tube, with respect to the electron beams in the present invention. In addition, a complicated process of dynamic convergence can be substantially elimated for a considerably large angle of deflection. A simple process of correction can take its place. Furthermore, the elongated slits formed in the control grid permit good use of the electron beams, thereby increasing the brightness of the color picture.

Accordingly, a primary object of the present invention is to provide a color television picture tube which can be produced at a low cost, substantially equaling the cost of producing a black-and-white television picture tube.

Another object of the invention is to provide a color picture television tube which enables a use an electron beam-deflecting device, which is substantially as simple in construction as the electron beam-deflecting device 'for a black-and-white television picture tube.

A still another object of the invention is to provide a color picture television tube which has an increased deflection sensitivity for electron beams.

A further object of the invention is to provide a color picture television tube capable of reproducing a color picture of high brightness.

Other objects of the invention as well as The features and advantages of the invention will become more apparent from the description set forth hereinafter when considered in conjunction with the accompanying drawings, in which:

FIG. IA shows the arrangement of electrodes in one embodiment of the color picture television tube constructed to this invention:

FIGS. 18 and 1C are views of optical equivalents showing the manner in which electron beams are formed in electron lenses in the embodiment of color picture television tube shown in FIG. IA;

FIG. 2A is an enlarged front view of a control grid in th embodiment of color picture television tube shown in FIG. 1;

FIG. 2B and 2C are enlarged views of electron beams formed in the electron lens of the embodiment of color picture television tube shown in FIG. 1;

FIGS. 3A and 3B are views schematically showing one example of the relative positions of a shadow-mask and the images of electron beams formed on a phosphor dot screen disposed in juxtaposed relation with the shadow-mask; and

FIGS. 4A and 4B are views schematically showing another example of the relative positions of a shadow-mask and the images of electron beams formed on a phosphor dot screen disposed in juxtaposed relation with the shadow-mask.

The color television picture tube according to this invention attains the aforesaid objects. It will now be explained with reference to the drawings.

The external shape of a color television picture tube embodying this invention is substantially similar to that of the conventional shadow-mask color television picture tube, so that its illustration is omitted. However, the essential portions of the inventive color television picture tube are shown in FIG. 1.

A phosphor screen 12 is composed of phosphors which are deposited on the inner side of a transparent faceplate 11 and housed in an evacuated glass bulb. A metal back 13, consisting of aluminum, is applied to the surface of the phosphor screen 12 by sputtering. Also housed in the evacuated glass bulb are cathodes 14, 15, and 16, a control grid 17 of special shape, a first anode (ring shape) 18, and a cylindrical second anode 19 making up a first electron lens L,, a cylindrical third anode 20, a cylindrical fourth anode 21, and a cylindrical fifth anode 22 making up a second electron lens L Taken together, the elements 14-22 constitute an electron gun.

A shadow-mask 23 is disposed in the neighborhood of the phosphor screen 12.

Horizontal and vertical deflection coils 24 provide a means for deflecting the electron beam emitted from the electron gun.

The horizontal and vertical deflection coils 24 are connected to horizontal and vertical deflection circuits 25.

It is to be observed that the arrangement of electrodes in the color television picture tube embodying This invention is substantially similar to that of a conventional black-and-white television picture tube, except for the shadow-mask 23 and the special shape of the control grid 17.

It is also to be understood that the resistors 39, 41 and 43 as well as the variable resistors and resistors making up a bleeder circuit are disposed outside of the glass bulb of the color television picture tube.

Currents with forms are fed to the deflection coils 24 from a vertical and horizontal deflection circuits 25. Thus, the electron beam emitted from the electron gun may be deflected to make the raster on the phosphor screen 12.

The application of voltages and signals to the electrodes for forming the color television picture tube will now be explained.

A high positive voltage is applied from a high voltage generator (not shown) through a terminal 26 to the phosphor screen 12.

This same generator is also connected to the ground of a television-receiving set through a series bleeder circuit composed of a resistor 27, variable resistor 28, resistor 29, variable resistor 30, variable resistor 31, variable resistor 32 and variable resistor 33. The shadow-mask 23 is connected to an adjustable contact 34 of the variable resistor 28. The third anode 20 and fifth anode 22 are electrically connected to each other in the interior of the glass bulb, and their junction point is connected to the junction point between the variable resistor 28 and resistor 29. The fourth anode 21 is connected to an adjustable contact 35 of the variable resistor 30. The first anode 18 is connected to an adjustable contact 36 of the variable resistor 31. The second anode 19 is connected to the junction point between the variable resistor 30 and variable resistor 31. The control grid 17 is connected to an adjustable contact 37 of the variable resistor 33. The cathode 14 is connected to an adjustable contact 38 of the variable resistor 32 through a resistor 39. The cathode 15 is connected to an adjustable contact 40 of the variable resistor 32 through a resistor 41. The cathode 16 is connected to an adjustable contact 42 of the variable resistor 32 through a resistor 43.

The brightness signal (Y) of a color television signal is applied from a video amplifier (not shown) through a terminal 50 to the control grid 17.

The red-yellow (R-Y) color difference signal is applied from a matrix circuit (not shown) through a terminal 47. The green-yellow (G-Y) color difference signal is applied from a matrix circuit (not shown) through a terminal 48. The blueyellow (B-Y) color difference signal is applied from a matrix circuit (not shown) through a terminal 49.

The shape of the control grid 17, which is one of the features of the present invention, will now be explained. As shown in FIG. 1A, the control grid 17 is formed with three elongated slits oriented vertically and disposed on the face thereof. Grid 17 juxtaposed with the electron-beam-emitting faces of the cathodes 14, 15 and 16. These focus are arranged in one plane which is in parallel relationship with the grid 17. These slits are a slit for red color 44, a slit for green color 45, and a slit for blue color 46. The use of these elongated slits in the present invention is conductive to increased brightness of reproduced color pictures.

The electron beams will now be described as it is emitted from the electron gun when voltages and signals are applied to the aforementioned electrodes, with reference to FIG. 1. Let us assume that the color difference signals (R-Y), (G-Y) and (B-Y) are equal in value.

Let us also assume that the (R-Y) color difference signal is applied to the cathode 14 through the terminal 47, the (G-Y) color difference to the cathode 15 through the terminal 48, and the (B-Y) color difference signal to the cathode 16 through the terminal 49, with the Y-signal being applied to the control grid 17. To focus an image, the potential of the first anode 18 is adjusted by moving the adjustable contact 36 of the variable resistor 31.

The adjustable contact 35, of the variable resistor 30, is moved to adjust the voltage applied to the cylindrical fourth anode 21, and control the focal length of the second electron lens L This lens is formed by the electric field created in the cylindrical anode 20, 21 and 22. This lens adjustment causes the electron image in the crossover point 52 of the electron beam 51, emitted from the cathodes, to be focused on the shadow-mask 23.

The electron beam 51 is focused on the shadow-mask 23. It has the smallest possible cross-sectional area corresponding, to that of the crossover point 52. This enables an increase in the ratio of the electrons in the beams passing through the apertures of the shadow-mask 23 as compared to the electrons in the beams prevented from passing therethrough. Accordingly, a large quantity of electrons strike the phosphor screen 12, thus increasing the brightness of high luminescence in a colored image on the television screen.

The electron beams 51 emanating from the cathodes 14, 15 and 16 are caused to move along paths shown by broken lines in FIG. 1A. These beams strike the phosphor. This movement is caused by a first electron lens L formed by an electric field between the first anode 18 and second anode 19, a second electron lens L formed by an electric field between the third, fourth, and fifth anodes 20, 21 and 22, and a third electron lens L formed by an electric field between the shadow-mask 23 and metal back 13.

An electron beam 51,. is produced by the (R-Y) color difference signal. An electron beam 51,, is produced by the (G- Y) color difference signal. An electron beam 51,, is produced by the (B-Y) color difference signal. These beams pass through the slit for red color 44, slit for green color 45, and slit for blue color 46 respectively. The images of these electron beams are formed on the faces of the respective slit. The first electron lens I. converge the beams on a crossover point 52, and focus them to form inverted images e, as shown in FIG. 2B. The image is in substantially the center portion of the lens. The inverted images 2 on the lens 1. are focused by the third electron lens L to form images e as shown in FIG. 2C, on the phosphor dot screen 12.

The third electron lens L relies on the known postdeflection focusing technique. The potential differential E, between the phosphor dot screen 12 and shadow-mask 23 is selected by actuating the adjustable contact 34 for the resistor 28. This adjustment is correct when the potential E, of the shadowmask satisfies the following relation:

EPEZ 3 E, This value constitutes a condition for forming an image of sufficiently high clarity of the phosphor dot screen 12. If the value is within this range, the electron beams will not deviate from the coated areas defined by the phosphor dots of three colors deposited on the inner surface of the face plate II.

FIGS. 13 and 1C are an equivalent optical showing of the manner of converging of the electron beams set forth hereinabove, FIG. 18 being a top plan view and FIG. 1C being a side view.

Accordingly, the crossover point 52 is focused in the third electron lens L and the images of electron beams formed in the second electron lens L are formed on the phosphor dot screen 12; then, the images of electron beams formed on the faces of slits 44, 45 and 46 respectively, can be formed on the phosphor dot screen 12. Thus, it is possible to produce a color picture on the color television screen if the electron beams 51,, 51, and 51, are intensity modulated by respective color video signals- The primary colors phosphor dots of the phosphor dot screen 12, are selected so that the phosphor dots struck by the electron beam 51, have a red color, the phosphor dots struck by the electron beam 51,, have a green color, and the phosphor dots struck by the electron beam 51,, have a blue color.

The construction and operation of the color picture television tube attains the set forth hereinabove. They will now be described in detail. As for the order of arrangement of the slits in the control grid 17, the slit for green color 44 is advantageously placed in the center and flanked by by the other two slits, as shown in FIG. 1A. This arrangement obtains better definition of the image. If a phosphor material of the sulfide system is employed for forming green phosphor dots on the phosphor dot screen, it is possible to obviate a reduction of luminescence caused by a saturation, due to the high density of electron beams. This reduction is accomplished by increasing the length of the slit for the green color 44 in the vertical direction, as shown in broken lines in FIG. 2A.

Alternatively, the control grid 17 may be divided into three segments. Each segment is formed with a slit for each of the three colors. Thus, the color difierence signals may be applied to the cathodes 14, 15 and 16, and the Y-signal may be applied to each segment of the grid.

The electron beams 51 emitted from an electron gun, including the control grid 17, pass through the first electron lens L which performs the function of minimizing the size of the point 52. This causes the images of electron beams formed on the faces of slits 44, 45 and 46 of the control grid 17 to be focused as clearly as possible in the second electron lens L Advantageously, the second electron lens is a composite electron lens made up of a number of electrodes. The crossover point 52 may be disposed in the second electron lens, depending on the construction of the later. However, this does not interfere with proper functioning of the color picture television tube.

The second electron lens L also focuses the image of the crossover point 52 on the shadow-mask 23. Preferably, the second electron lens L. has a construction which corresponds to the construction of a large aperture lens, of the large thickness type, which is free from distortions in the periphery. The electron beams 51, passing through this second electron lens L, have a very small cross-sectional area. No dynamic, convergence is required, as is the case with the conventional black-and-white television picture tube.

Spots of higher accuracy are provided in the periphery of the picture by effecting a correction by a simple process. The images e, formed in the second electron lens L, by the electron beams 51,, 51, and 51,, may be out of focus. If so, slits are formed in a metal plate (not shown) provided in the entrance to, or the exit from the center of the second electron lens L so that the images e may be formed on the slits.

These slits may be of the same shape as the slits of the control grid 17 (shown in FIG. 2A), but they are preferably of the same size as the slits shown in FIG. 28. Care should be taken so that the amount of secondary electrons and scattered electrons produced by the electrons striking the metal slit may be minimized. The secondary and scattered electrons should not strike the phosphor clot screen 12. The slits may, for example, be configured so that their lateral surfaces are tapered in cross section to be open on the side facing the phosphor dot screen I2. Alternatively, an electrode may be provided for catching secondary electrons on each lateral side.

The electron beams 51,, 51,, and 51,, pass through the second electron lens L In this way, the beams are deflected vertically and horizontally by the deflection coils 24, and they are directed toward the shadow-mask 23, to pass; through the apertures 53 therein. The construction of the shadow-mask 23 and phosphor dot screen 12 will be explained in detail hereinafter.

The apertures 53 are arranged in the shadow-mask 23 as shown in FIG. 3A, for example. Phosphor dots P,., g and P of the three primary colors, are disposed on the face plate 11 and in positions corresponding to the positions of apertures 53. The phosphor dots are applied to the face plate by the electron-beam-printing process. There is a deviation of the electron beams caused by the vertical component I-Iv of the earth s magnetism when the picture tube is in operation. This deviation is taken into consideration when producing the phosphor dot screen. FIG. 3B shows an example of the phosphor dots formed by this process. A disagreement may arise between the electron beams 51 and the phosphor dots if the electron beams are subjected to an up or down deviation by the horizontal component I-Iv of the earths magnetism. Thus disagreement may exert a bad influence on the performance of the color picture television tube depending on the locations in which it is used.

To obviate this problem, one has only to impart to the electron beams, an 'up-and-down vibration of :tH which is greater than the horizontal component of force Hh. It is also possible to increase the number of apertures 53 of the shadow-mask 23 in performing electron beam printing of phosphor dots.

FIGS. 4A and 4B show phosphor dots formed in this manner. As clearly seen from these figures, the phosphor dots P,, P, and P, are arranged in substantially a row. An increase in the value of iI-I will cause them to be completely arranged in a row to be disposed on a straight line. If the phosphor dots are formed in this way, the color picture tube will substantially impervious to the influences of the earths magnetism so long as the television set is positioned substantially horizontally while in operation. Moreover, the increase in the number of the apertures 53, results in an increase in the rate of transmission of the electron beams through the apertures.

The aforementioned voltage is then applied to the electric field between the shadow-mask 23 and metal back 13. This voltage, causes the images of electron beams in the second electron lens to be formed on the phosphor dot screen 12 so as to thereby produce images e' shown in FIG. 2C.

One of the problems involved in postacceleration of this type is a degradation of contract of an image reproduced on the screen of the color television picture tube. This phenomenon is mainly due to the fact that the phosphor dots are struck by reflected electrons, scattered electrons, secondary electrons, and the like. These electrons are produced by and passed through the apertures 53 when the electron beams impinge on the shadow-mask 23. In order to obviate this problem, the'apertures 53 of the shadow-mask may be shaped so that the sidewalls thereof are diverged in cross section toward the phosphor dot screen 12. This divergence prevents the electron beams 51 from impinging on the periphery of the apertures 53 FIG. 1A.

In addition, a coat of material which can prevent the reflection of scattered electrons. This material and secondary electrons may be applied at the entrance to the sharp-angled portion of each aperture 53. In this manner, the number of scattered electrons and secondary electrons can be reduced. The degradation of contrast of an image reproduced on the screen of the color picture television tube can be obviated.

Beams of reflected electrons, scattered electrons, and the like, may be produced when electron beams 51 pass through the metal back 13 and excite the phosphor dots on the phosphor dot screen 12. To prevent these electrons, care is exercised in determining the thickness material of the metal back 13 and in selecting a proper process for applying phosphor dots. It will thus be possible to cause the images e to focus on the phosphor dots and to cause the phosphor dots to emit colored light of high brightness.

It will thus be evident that the present invention obviates all the disadvantages of conventional color television picture tubes. The images 2 are not spot shaped, but are similar to the images of electron beams formed on the faces of the vertically oriented elongated slits of the control grid. When struck by vertically elongated electron beams, the brightness of the phosphor dots is much higher than that of the phosphor dots which are struck by spot-shaped electron beams, if the degree of saturation may be constant.

The color television tube constructed according to this invention has an increased deflection sensitivity because the electron beams are very small in cross section area. Also this is true because the picture tube is of the postdefiection focusing type.

The present invention has been described as being embodied in a color television picture tube of the three-color type. However, it is to be understood that the invention is not limited to the precise form of the embodiment described.

The invention can be embodied in color television picture tubes of a two-color type or more than three colors. Other changes and modifications which are obvious to persons skilled in the art can also be made in the embodiment without departing from the scope and spirit of the invention.

What I claim is:

l. A color television picture tube comprising a single electron gun including a plurality of cathodes arranged in one plane and parallel to one another for emitting electron beams, a control grid formed with a plurality of vertically elongated slits disposed in positions corresponding to the positions of said cathodes, at least one anode, and a cascaded series of cylindrical electrodes; a phosphorescent screen consisting of a plurality of colored dots; and apertured shadow-mask interposed between said electron gun and said phosphorescent screen; first electron focusing means for converging the electron beams emanating from said cathodes to a crossover point and focusing images of the electron beams of the faces of said slits in saidcylindrical electrodes; second electron focusing means for focusing said crossover point on said shadow-mask; and third electron focusing means for reforming the images of electron beams focused in said cylindrical electrodes on said phosphorescent screen, said plurality of dots being coated with different color phosphors on the positions of the reformed images corresponding to each of said slits of the control grid.

2. A color television picture tube as defined in claim 1 in which said first electron focusing means includes means for controlling a voltage applied to said at least one anode; in which said second electron focusing means includes means for controlling voltages applied to said cylindrical electrodes; and in which said third electron focusing means includes means for controlling a voltage applied to said shadow-mask.

3. A color television picture tube as defined in claim 1 in which said plurality of cathodes are three in number; in which said control grid is formed with three vertically oriented elongated slits corresponding to three primary colors arranged in side-by-side relation and juxtaposed to the electron-emanating faces of said cathodes respectivelyl, the slit for green color being disposed in the center to be anked by the slits for red and blue colors; and in which said plurality of dots are coated with three primary color phosphors on the positions of the reformed images corresponding to each of said slits of the control grid.

4. A color television picture tube as defined in claim 3 in which said slit for green color has a longer vertical length than the other slits. 

1. A color television picture tube comprising a single electron gun including a plurality of cathodes arranged in one plane and parallel to one another for emitting electron beams, a control grid formed with a plurality of vertically elongated slits disposed in positions corresponding to the positions of said cathodes, at least one anode, and a cascaded series of cylindrical electrodes; a phosphorescent screen consisting of a plurality of colored dots; and apertured shadow-mask interposed between said electron gun and said phosphorescent screen; first electron focusing means for converging the electron beams emanating from said cathodes to a crossover point and focusing images of the electron beams of the faces of said slits in said cylindrical electrodes; second electron focusing means for focusing said crossover point on said shadow-mask; and third electron focusing means for reforming the images of electron beams focused in said cylindrical electrodes on said phosphorescent screen, said plurality of dots being coated with different color phosphors on the positions of the reformed images corresponding to each of said slits of the control grid.
 2. A color television picture tube as defined in claim 1 in which said first electron focusing means includes means for controlling a voltage applied to said at least one anode; in which said second electron focusing means includes means for controlling voltages applied to said cylindrical electrodes; and in which said third electron focusing means includes means for controlling a voltage applied to said shadow-mask.
 3. A color television picture tube as defined in claim 1 in which said plurality of cathodes are three in number; in which said control grid is formed with three vertically oriented elongated slits corresponding to three primary colors arranged in side-by-side relation and juxtaposed to the electron-emanating faces of said cathodes respectively, the slit for green color being disposed in the center to be flanked by the slits for red and blue colors; and in which said plurality of dots are coated with three primary color phosphors on the positions of the reformed images corresponding to each of said slits of the control grid.
 4. A color television picture tube as defined in claim 3 in which said slit for green color has a longer vertical length than the other slits. 